أعدى الزمان سخاؤه
His generosity communicated its character to time.
Aʿdā implies the transfer of a condition or quality, not simply the giving of a gift. Generosity spreading to time, and time acting generously or miserly, are figurative predications.
The logic of the verse depends upon a sequence of cause and effect: the patron's generosity, time's supposed generosity, and the possibility of a contrasting description of time.